from Hacker News

People inside might have RFID / NFC technology with them, like badges, but we can't count on them 100% as someone might forget or not have that piece at all.

Of course, buses will slow down when they come to a "checkpoint" to allow devices (cameras) to perform better scanning.

By the way, it's a civil project, nothing to do with law enforcement. A huge convention center wants to know in advance, if 100 buses are coming, what number of participants to expect at their gate.